In offset printing, the printing plate is generally made of aluminum or stainless steel in thin sheet form. The plates are treated photographically or otherwise to form greasy image regions thereon. The non-image portion of the plate is essentially bare metal. The printing plates are bound and tightly clamped around a cylinder generally termed a "printing cylinder". In use, the plate is "dampened" by applying, through a dampener roller or other dampener system, a thin sheet of water to the plate with each revolution of the printing cylinder. Greasy inks are then applied to the plate, usually by means of a roller in contact with the printing cyclinder. The greasy ink is attracted to the greasy image regions of the plate, and is repelled by the water-sheeted bare metal portions of the plate. Similarly, the water applied to dampen the plate is attracted to and sheets over the bare metal portions thereof but is repelled by the greasy image portions thereof.
The dampening water discussed above is normally actually a solution, commonly termed a "fountain solution" because it is carried on the press in a fountain pan, and is not simply water but a solution with additives therein designed to increase its effectiveness. The most common fountain solutions in general use contain small amounts of gum arabic (acacia), and chromic acid or phosphoric acid. Such solutions are often termed "gum and etch" solutions. In many instances alcohol, usually isopropyl alcohol, is included in the fountain solution to improve the wetting qualities.
The functions which must be performed by a fountain solution include thoroughly and uniformly wetting the bare metal portion of the plate while at the same time not wetting the greasy image portion of the plate, keeping the plate clean during the press run, and dissolving any dirt, especially grease-attracting dirt, which finds its way on to the bare metal portions of the plate. If these functions are not performed effectively, the result is a splotchy, dirty-appearing blemished background in the printed product. If the printed product is not satisfactory, paper, ink, and press time are thus wasted and costs are increased.
The sources of dirt finding its way on to the plates include the ink and paper employed in the press, and the naturally dirty atmosphere of a pressroom. Furthermore, the plate operates in an inherently corrosive environment since it is repeatedly coated with water and then wiped clean and exposed to the atmosphere in the course of printing operations. Stainless steel, and to a lesser extent aluminum, are commonly considered corrosion resistant, but in the severe application of offset printing operations, they are both actually quite susceptible to corrosion.
The function of the acid in the conventional gum and etch solutions is to eat away spots of corrosion on the plates, and to dissolve other dirt. The function of the gum arabic is to supply a thin unobjectionable protective coating to the plate. The function of the alcohol, as noted above, is to improve the wetting qualities of the solution.
Conventional gum and etch solutions present a number of problems. If the acid component is not carefully controlled, the material of the plate, as well as corrosion spots, may be dissolved to some extent, thus roughening the plate and causing the very blotchiness which the fountain solution is supposed to prevent. In addition, the acid component may attack the greasy image portion of the plate which also decreases the quality of the printed product. Gum arabic is a natural product and its quality and functional properties naturally vary from lot to lot in unpredictable manners, making continual experimentation at the press necessary to produce a satisfactory fountain solution for operations. Futhermore, since gum arabic is a natural product, its supply and its price vary objectionably. In particular, the price has increased several fold in recent times, thus pushing up printing costs undesirably.
Another problem that occurs in offset printing operations is that of storage of printing plates prior to and between press runs. The plates are normally removed from the press if they are not going to be used in the very next press run. On the other hand, they may be stored overnight or over a weekend by being left on the press. In any event, the plates are inherently very sensitive to corrosion and collection of dirt from the press room atmosphere during storage. Corroded and dirty plates are sometimes rendered unusable, and must be replaced. In addition, plates taken out of storage often require considerable rehabilitation work to remove corrosion and dirt before they can be put back on the presses. Furthermore, when put back on the presses, such plates often require some tinkering with and adjusting of fountain solutions, etc., before they can be made to print in a satisfactory manner. All of such work is time consuming and costly.
It is customary to prepare plates for storage by applying a rub-up solution similar in composition to fountain solution discussed above, but normally somewhat more concentrated. Other gum arabic based formulations are also used. The object of such rub-ups is to apply a thin protective coating of gum arabic to the plate. Such a coating is intended to prevent corrosion and to shield the bare metal from ambient dirt.